Heretofore, as a method for producing DL-phenylalanine from an .alpha.-halogeno-.beta.-phenylpropionic acid, there has been known a method in which aqueous ammonia containing the .alpha.-halogeno-.beta.-phenylpropionic acid is reacted in an autoclave at room temperature or under heating (C. S. Marvel, Org. Synth, Coll. Vol. 3, p705 (1955); A. M. Yurkevich et al., Zhur. Obshchei Khim. 28, 227 (1958)).
However, according to this method, if the reaction is conducted under heating, the yield of DL-phenylalanine is as low as from 40 to 60%, and if the reaction is conducted at room temperature, it takes about 1 week until the reaction is complete. Neither case is practical.
On the other hand, it is also known to conduct the reaction by using liquid ammonia in an autoclave at room temperature for a long period of time (R. Filler et al., Proc. Chem. Soc. 1962, 117; Can. J. Chem., 45, 325). However, the recovery of liquid ammonia is not easy, and thus the process has a problem from the industrial point of view.
The present inventors have conducted extensive researches for the process for producing DL-phenylalanine from an .alpha.-halogeno-.beta.-phenylpropionic acid, and have found that the higher the molar ratio of ammonia to the .alpha.-halogeno-.beta.-phenylpropionic acid in the reaction solution is, the higher the yield of DL-phenylalanine becomes. However, this process requires ammonia in great excess, and it has been found that in order to attain a good yield, e.g. about 80%, ammonia is required to be employed in an amount as much as about 40 times the stoichiometric amount. Thus, this process has difficulties in the volume efficiency of the reactor and the recovery of ammonia.
From further researches, the present inventors have found that when the reaction is conducted while supplying the .alpha.-halogeno-.beta.-phenylpropionic acid into a solution containing ammonia charged in the reactor in advance, it is possible to produce DL-phenylalanine in good yield even if the molar ratio of ammonia to the .alpha.-halogeno-.beta.-phenylpropionic acid is substantially lower than that in the conventional techniques. The present invention has been accomplished based on this discovery.